Concrete floors may be considered under two classes, one in which the concrete in combination with steel members forms the whole of the floor, and the other in which steel I-beams are set as for hollow tiles with the spaces filled with concrete upon wire cloth or expanded metal, or with these or strands of wire embedded, as tension members, in a plate or arch of concrete.

Of the first-named class of concrete floors, the Ransome patent is the best known. (Fig. 218.) This consists of the use of twisted square bars of steel running through the lower portion of the plate or beam to give the necessary tensile strength to the concrete; it has been used for spans up to thirty or forty feet without steel beams or girders. Another form of this floor consists of a lighter construction of concrete beams each with its twisted steel member, with a thin plate of concrete between. (Fig. 219.)

Fig. 218. Heavy Ransome Floor.

The other system of concrete floor construction differs from the hollow tile floor construction in the use of concrete for a filling between the beams instead of tiles; steel columns, girders and beams remaining the same as for the tile floors.

These floors are usually to be found in one of two forms - the flat plate of concrete lying between the beams or the segmental arch of concrete formed upon a center of metal lathing. Of the former system, the plate floor of the Expanded Metal Co. is a good example. (Fig. 220.) This floor consists of a plate of cinder concrete, from three to seven inches thick, in which is embedded a continuous sheet of expanded metal which is laid over the tops of the beams and is allowed to sag down between them a few inches. A flat centering of planks is hung with its surface a little below the sag of the metal, and a concrete of cinders and Portland cement is poured over and through the meshes of the expanded metal, and leveled off at an established height above the tops of the steel floor beams. Troughs are formed in the centering at each side of the floor beams which allows the concrete to completely enclose the beam.

Fig. 220. Expanded Metal Floor.

On this plate of concrete, a wooden or cement floor may be laid in the usual manner, and a flat ceiling formed below by hanging stiffened metal to the beams; or the underside of the concrete plate may be plastered upon, forming a series of panels. These plates, of seven inches thickness have been used up to seventeen feet of span between the beams; and the average floor will weigh about thirty-, five pounds to the square foot, and is about four inches thick.

Of the types of arched concrete floors, the Roebling patent is a fair example. (Fig. 221.) This construction consists of an arch of wire cloth stiffened with rods, which is sprung between the floor beams with more or less of a rise according to the spacing of the beams. Upon this arch, a concrete of Portland cement and sand is deposited, making, when set, a solidslab of concrete three inches or more in thickness at the top, with the haunches leveled up to the top of the arch. The most economical proportions for this floor have been found to be a basis of ten-inch beams, spaced according to the span and load, with an arch of three or four inches at the crown. This makes a very strong floor, and one not easily damaged, as it may be punctured by holes of any size without destroying the concrete arch, which is kept from shattering by the wire cloth. If a flat ceiling is required, it may be hung to the bottom of the beams of stiffened wire cloth and may be flat or panelled as desired. No centering is required with this floor, as the stiffened arches are bent to the required curvature, and after being set in place, they possess enough strength to receive the load of concrete or the shock of any falling body. Being open to the air both above and below, the concrete sets quickly, two days being generally enough for safe use.

Fig. 221. Roebling Floor.

While these systems of floor construction are typical of the main features of all, they are not necessarily the best or only kinds, for many other systems, embodying variations of these features, are in constant use, some making use of twisted wire strands in place of expanded metal to support the concrete plate, others, steel bars of special pattern.

Selection

With so many styles of floor available, each possessing merits of its own, the architect will often be at a loss to decide upon a special system. Cost, which is an ever-present factor, will decide in some cases, and in others, local considerations may require particular methods. In the main, consideration should be given to lightness, strength, and fire-resisting qualities, as well as speed of erection and the skill of available workmen. Of the respective merits of the two rival systems of terra-cotta arches and concrete, much has been written, especially in regard to fire-resisting qualities, but the question of durability cannot be said to have been settled at the present time. For very high buildings one of the systems requiring the use of steel floor beams and girders will usually be adopted, and the beams spaced with reference to the load and the requirements of the floor construction; these beams may rest entirely on the top of the girders, but are usually framed flush with the top, allowing the girder to project if it is deeper than the beam. The beams are connected with the girder by means of angles riveted to the webs of the beam and girder. (Fig. 222.) Connections of the floor beams with the columns, which will occur where the spacing of the floor beams brings a beam opposite to a column, are made as described for light girders. The outside beams of the floor are sometimes allowed to run behind the exterior columns, forming no part of the exterior construction, but more often they are framed between the columns, and, in connection with other supports, help to carry the enclosing walls of the building. (Fig. 223.)